Thud noise when cadence is set

Have a hybrid triple 8 speed Tiagra rear derailuer new installed, bike less then 1 year old.

Once excelerating to a constant speed or cadence i get on my downstroke a Thud or hammer noise, as if the chain is loose and catches the cassette teeth, just like when you about to start pedalling.
This is the best that i can describe the problem.
This hesitation or hickup or thump is very annoying and is present when i'm at a set cadence when pedalling. Another analogy is if you skipped a gear but happens at every down stroke when pedalling the cranks. Its like trying to open a combination lock you pull and pull but it won't open.
This problem came just after the bike was taken to the LBS. When you turn the bike over and turn the cranks it all looks good and smooth but it's a different story when riding on the street.

I'm i imagining this or is it a problem that can be fixed like a loose chain? How would a chain get loose in the first place.

No chain replacement still original, got into a accident frame was fixed maybe when car hit me, the back wheel tore out of the frame the chain got damaged?
But the LBS should know that?
The chain looks okay, but with closer examination, on a smaller gear ring i see the chain ripple or flutter is that normal?
Whats a stiff link meaning what would cause it to tighten up?

"but with closer examination, on a smaller gear ring i see the chain ripple or flutter is that normal?"
Nope! That indicates a stiff link.
Another way to test is slowly rotate the crank backwards and observe the jockey pulleys on the RDER. If you see them twitching or jumping, that's where your stiff link(s) is/are.
You can normally remove the stiffness by applying a bending force perpendicular to the direction of chain flex. Push both directions and test. Repeat with a bit more pressure if/as necessary.

LBS told me it could be from rear hub. The hub has 16 gears for the crank to start to grip and spin. To me the rear wheel is new but Alex DA 17 from my old DA 16's.
This gives me a good idea of why i get that thud or slip when pedalling.
Any ideas of correcting the problem?